Lifting puzzles and congruences of Ikeda and Ikeda-Miyawaki lifts

摘要

We show how many of the congruences between Ikeda lifts and non-Ikeda lifts, proved by Katsurada, can be reduced to congruences involving only forms of genus 1 and 2, using various liftings predicted by Arthur's multiplicity conjecture. Similarly, we show that conjectured congruences between Ikeda-Miyawaki lifts and non-lifts can often be reduced to congruences involving only forms of genus 1, 2 and 3.